Steel section having a thickness of at least 100mm and method of manufacturing the same

ABSTRACT

A steel section has a web central portion connected on each side to a flange portion having a thickness of at least 100 mm. The steel section microstructure includes at least one kind of vanadium precipitates possibly comprising also one or more metal chosen among chromium, manganese and iron, the precipitates being chosen among nitrides, carbides, carbo-nitrides or any combination of them, more than 70% of such precipitates having a mean diameter below 6 nm. It also deals with a manufacturing method thereof.

The present invention deals with a steel section comprising a webcentral portion connected on each side to a flange portion having athickness above 100 mm. The steel section according to the invention isparticularly well suited for the manufacture of columns for high-risebuildings, long span, transfer and belt trusses, outriggers and bridgegirders.

BACKGROUND

The development of new modern structural steel grades is always drivenby the users' requirements towards higher mechanical properties such asyield strength and toughness, as well as excellent technologicalproperties, ensuring an efficient fabrication technology at workshop andon site.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a steel heavy sectionreaching a high yield strength of at least 485 MPa and a high tensilestrength of at least 580 MPa with excellent weldability.

In the practice of structural steel manufacturing, it is known that inorder to improve strength and toughness it is preferable to refine thestructure through hot rolling at lower temperatures or to add somealloying elements for austenite grain refining. Both solutions are notsufficient for heavy structural steel manufacturing, because in case oflower hot rolling temperatures the overheating of the rolls isinevitable. At the same time, when the alloyed elements are added inhigh amounts, the weldability of the steel deteriorates.

It is an object of the present invention to provide a steel section,comprising a web central portion connected on each side to a flangeportion having a thickness of at least 100 mm, such steel section havinga composition comprising, in weight percentage:

-   -   C: 0.06-0.16%    -   Mn: 1.10-2.00%    -   Si: 0.10-0.40%    -   Cu: 0.001-0.50%    -   Ni: 0.001-0.30%    -   Cr: 0.001-0.50%    -   Mo: 0.001-0.20%    -   V: 0.06-0.12%    -   N: 0.0050%-0.0200%    -   Al≤0.040%    -   P≤0.040%    -   S≤0.030%    -   and comprising optionally one or more of the following elements,        in weight percentage:    -   Ti<0.005%    -   Nb≤0.05%    -   the reminder being iron and impurities resulting from        elaboration, and said steel section microstructure including at        least one kind of vanadium precipitates possibly comprising also        one or more metal chosen among chromium, manganese and iron,        said precipitates being chosen among nitrides, carbides,        carbo-nitrides or any combination of them, more than 70% of such        precipitates having a mean diameter below 6 nm.

Another object of the present invention is to provide a method ofmanufacturing of a steel section comprising the following steps:

-   -   feeding a steel semi-product which composition is according to        claims 1 to 3,    -   reheating such steel semi-product at a temperature above        1000° C. and hot rolling it with a final rolling temperature of        at least 850° C., to obtain a hot rolled steel section,    -   cooling the hot rolled steel section so as to produce        martensitic and/or bainitic quenching of the surface layer of        all or part of the product, the non-quenched portion of the        rolled product remaining at a temperature high enough to make it        possible to cause a self-tempering of the quenched surface layer        of martensite and/or bainite and to transform the austenite into        ferrite and carbides in the core part of the section during the        subsequent cooling, the maximum temperature of the tempered        surface of the product after quenching being 450 to 650° C.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention andthe drawings:

FIG. 1: shows an electron micrograph illustrating randomly distributedprecipitates in the core of the flange of the heavy section,

FIG. 2: shows an electron micrograph illustrating precipitates, arrangedin regularly spaced bands, and

FIG. 3: shows schematically the web portion and flanges of the steelsection.

DETAILED DESCRIPTION

All compositional percentages are given in weight percent (wt. %),unless indicated otherwise. Regarding the chemical composition of thesteel, carbon plays an important role in the formation of themicrostructure and reaching of the targeted mechanical properties. Itsmain role is to provide strengthening through hardening of themartensite/bainite phases but also through formation of carbides and/orcarbo-nitrides of metallic elements of the steel. The carbon content ofthe grade according to the invention is between 0.06 and 0.16% weight.Carbon content below 0.06% will not result in a sufficient level ofmechanical resistance, leading to yield strengths value below 485 MPa.On the opposite, carbon contents above 0.16% would result in reducingductility and the weldability of the steel. Preferably, the carboncontent is between 0.08 and 0.14%, so as to obtain sufficient strengthand weldability.

Manganese is an element which increases hardenability. The manganesecontent of the grade according to the invention is between 1.10 and2.00%. Manganese content below 1.10% will not result in a sufficientlevel of mechanical resistance. On the opposite side of the range,manganese content above 2.00% would result in decreased weldability orwould promote the formation of hard martensite-austenite constituents,also negatively impacting the toughness of the steel.

Silicon is a deoxidizing element and contributes to improving strength.Silicon content below 0.10% will not result in a sufficient level ofmechanical resistance nor a good deoxidation. On the opposite side ofthe range, silicon contents above 0.40% would result in the formation ofoxides, reducing welding properties of the steel.

Copper is an element contributing to improving the strength of the steelby hardenability improvement and precipitation strengthening. Coppercontent below 0.001% will not result in a sufficient level of mechanicalresistance. On the opposite side of the range, copper contents above0.50% would result in increasing the carbon equivalent and thusdeteriorating the weldability or impacting the hot shortness of thesteel during hot deformation, caused by penetration of the Cu-enrichedphase into grain boundaries.

Nickel is an element contributing to improving the strength andtoughness of the steel. Nickel content below 0.001% will not result in asufficient level of mechanical resistance. On the opposite side of therange, nickel contents above 0.30% would lead to high alloying costs.

Chromium is an element contributing to improving the strength of thesteel by improving hardenability through solution hardening but alsothrough precipitation hardening. Chromium content below 0.001% will notresult in a sufficient level of mechanical resistance. On the oppositeside of the range, chromium contents above 0.50% would result ingenerating coarse chromium carbides or carbo-nitrides that maydeteriorate the toughness of the steel

Molybdenum is an element contributing to improving the strength of thesteel by improving hardenability. Molybdenum content below 0.001% willnot result in a sufficient level of mechanical resistance. On theopposite side of the range, molybdenum contents above 0.20% would resultin reducing the toughness of the steel.

Vanadium is an important element that is used to achieve hardening andstrengthening by precipitation of nitrides, carbo-nitrides or carbidesbut also through grain refining. The formation of vanadium precipitationlimits the austenite grain coarsening, by resulting in ferrite graindecrease and improved strength by precipitation in ferrite phase.Vanadium would also prevent the chromium and manganese migration in thecementite, resulting in their application in small precipitationformation. Vanadium content below 0.06% will not result in a sufficientlevel of mechanical resistance. On the opposite side of the range,vanadium contents above 0.12% would result in a risk that an excessiveprecipitation may cause a reduction in toughness, which has to beavoided. In a preferred embodiment, vanadium addition is limited to0.09% to improve further the toughness of the steel.

Nitrogen is an important element to form nitrides and carbo-nitrides ofmetallic elements like vanadium, niobium aluminum and titanium. Theirsize, distribution density and stability have a significant effect tomechanical strengthening. Nitrogen content below 0.0050% will not resultin a sufficient level precipitation and grain size control. To furtherimprove those properties, a minimum level of 0.0060%, or even of 0.0070%or even better of 0.0080% is preferred. On the opposite side of therange, nitrogen contents above 0.0200% would result in the presence offree nitrogen in the steel, which is known as having a negative impacton toughness in the Heat Affected Zone after welding.

During hot rolling, part of the vanadium will combine with nitrogen inorder to form VN particles for austenite grain boundaries pinning. Theremaining vanadium, in solution, will then precipitate in form of fineprecipitates during cooling of the steel, thus making an importantcontribution to final strength. The inventors have found that theprecipitation strengthening can be enhanced by optimizing the vanadiumto nitrogen ratio in the steels section to approach the stoichiometricratio of 4:1. In a preferred embodiment, the ratio of V to N iscomprised between 2.5 and 7, and even comprised between 3 and 5.

Aluminium can be added in the steel for deoxidizing effect and removingof the oxygen from the steel. If other deoxidizing elements are added inthe steel, the aluminum content is 0.005% and lower. Otherwise, thealuminum content is between 0.005% and 0.040%. If the aluminum contentis too high, the formation of AlN will occur in preference to VN, andAlN being bigger in size than VN, it will be not as efficient forpinning of austenite grain boundaries as VN.

Sulfur and phosphorus are impurities that embrittle the grain boundariesand lead to the formation of center and micro-segregation. Theirrespective contents must not exceed 0.030 and 0.040% so as to maintainsufficient hot ductility and to avoid deterioration in weldingproperties.

Niobium is an element that may optionally be used to achieve hardeningand strengthening by precipitation of nitrides, carbo-nitrides orcarbides. It suppresses the growth of austenite grains during rolling,by refining them, thus resulting in improvement of strength andlow-temperature toughness. However, when its amount is above 0.05%,could deteriorate toughness in the Heat Affected Zone due to martensitehardening. On the other hand, when niobium amount is 0.05% and higher,it will pin to available nitrogen and thus impairing nitrogen fromforming vanadium precipitates that assures the strengthening of theductile core of the section.

Titanium is an element that may optionally be used to achieve hardeningand strengthening by precipitation of nitrides, carbo-nitrides orcarbides. However, when its amount is above or equal to 0.005%, there isa risk of TiN formation rather than VN. Moreover, TiN being cuboidsparticles may react as stress concentrators thus negatively impactingthe toughness and fatigue properties of the steel. In a preferredembodiment, the maximum amount of titanium is set to 0.003% and even to0.001%.

In a preferred embodiment, the carbon, manganese, chromium, molybdenum,vanadium, nickel and copper contents of the grade are such that

0.4≤CEV≤0.6 with CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15

Respecting these values ensures that the hardenability of the steelsection will be in suitable ranges through sufficient formation ofbainite, while maintaining a good weldability of the steel sections. Thereduced carbon equivalent allows avoiding weld processing steps such aspreheating (when acceptable) and also results in reduction offabrication costs. In a preferred embodiment, CEV≤0.5%.

The steel section comprises a web central portion 100 connected on eachside to a flange portion 102, 104, as shown schematically in FIG. 3.

The thickness of the flange of the steel section according to theinvention is set above 100 mm, allowing the use of such beam forhigh-rise building structures, notably. Its thickness is preferablybelow 140 mm as a sufficient cooling rate to ensure the requestedtensile and toughness properties is difficult to obtain.

According to the invention, the web and the flanges of the heavy sectionare composed of a hardened zone, resulting from the water cooling of thesurface and a non-hardened zone, in the core of the product. Each zoneof the steel section can have a specific microstructure that can includeone or more phases among tempered martensite, bainite, ferrite andpearlite. Ferrite can be present under the form of acicular ferrite orof regular ferrite.

The microstructure of each zone depends on the steel section thicknessand on the thermal path it is subjected to.

In a preferred embodiment, the microstructure of the flanges portionsinclude, from surface to core, a first zone comprising temperedmartensite and possibly bainite and a second zone comprising ferrite andpearlite.

The first zone can, for example, extend up to 10 mm under the surface ofthe flange portion.

An essential characteristic of the invention is the presence, in thesteel section microstructure, of at least one kind of vanadiumprecipitates possibly comprising also one or more metal chosen amongchromium, manganese and iron, said precipitates being chosen amongnitrides, carbides, carbo-nitrides or any combination of them, more than70% of such precipitates and preferably more than 80%, having a meandiameter below 6 nm. The mean diameter determination was done in thefollowing way: the surface of each detected precipitate was measured andapplied to the corresponding circle, from which the diameter wasextracted, giving then the mean diameter size for all detectedprecipitates.

In a preferred embodiment, the mean density of those precipitates is ofat least 500 precipitates per mm², preferably of at least 1000precipitates per mm². Those precipitates have a beneficial effect onstrength, known as being increased with precipitates size decrease andprecipitates content increase.

Such precipitates are preferably present in the core zone of the flangeof the section, mainly in the ferrite phase. At least 70% of suchprecipitates and preferably at least 80%, have a mean diameter below 6nm. The reduced size of such precipitates increases their hardeningeffect and hence the tensile strength of the steel section.

In a preferred embodiment, two types of precipitates are preferablypresent in the core of the flange of the steel section:

-   -   precipitates randomly distributed inside ferrite and    -   precipitates arranged in regularly spaced bands, forming thus        parallel sheets densely populated with particles.

The randomly distributed precipitated are bigger than the one arrangedin regularly spaced bands.

In a preferred embodiment, such regularly spaced precipitates include atleast vanadium and chromium.

In another preferred embodiment more than 80% of the randomlydistributed precipitates have a mean diameter between 3.5 and 6 nm. Suchprecipitates preferably include at least vanadium, chromium and iron.

The steel section according to the invention can be produced by anyappropriate manufacturing method and one of skill in the art can defineone. It is however advisable to use a process ending by an acceleratedcooling, in that case quenching and self-tempering of the surface layerafter hot-rolling step.

The method according to the invention comprises the following steps:

-   -   feeding a semi-product which composition is according to the        invention    -   reheating such semi-product at a temperature above 1000° C. and        hot rolling it with a final rolling temperature of at least 900°        C., to obtain a hot rolled steel section,    -   cooling the hot rolled steel section so as to produce        martensitic and/or bainitic quenching of the surface layer of        all or part of the product, the non-quenched portion of the        rolled product remaining at a temperature high enough to make it        possible to cause a self-tempering of the quenched surface layer        of martensite and/or bainite and to transform the austenite into        ferrite and carbides in the core part of the section during the        subsequent cooling, the maximum temperature of the tempered        surface of the product after quenching being 450 to 650° C. and        even 550-650° C.

The steel sections according to the present invention are preferablyproduced through a method in which a semi product made of a steelaccording to the present invention having the composition describedabove, is cast, the cast input stock is heated to a temperature above1000° C., preferably above 1050° C. and more preferably above 1100° C.or 1150° C. or used directly at such a temperature after casting,without intermediate cooling. Such temperatures allow full dissolutionof vanadium carbonitrides, which will further participate inprecipitation strengthening mechanism.

The final hot-rolling step is performed at a temperature above 850° C.The end-of-rolling temperature is above or equal to 850° C. in order toassure the austenite grains refining and thus the formation of a thinnermicrostructure after transformation, which is known to enhance thetoughness and strength properties.

During hot-rolling, it is preferable to use managed combination ofrolling steps and controlling the rolling temperature. The aim is tocreate fine grained microstructure by grain refinement during thesubsequent recristallization during rolling.

The hot-rolled product obtained by the process described above is thencooled using preferably a quenching and self-tempering process.

The so-called quenching and self-tempering process (QST) consists insubjecting a hot rolled steel section emerging from the finishing standof the rolling mill to cooling by means of a fluid so as to producemartensitic and/or bainitic quenching of the surface layer of all orpart of the product. Moreover, at the outlet of the fluid cooling zone,the non-quenched portion of the rolled product is at a temperature highenough to permit, during subsequent air cooling, tempering of thesurface layer of martensite and/or bainite to take place.

The cooling fluid employed for carrying out the quenching and selftempering step is usually water with or without conventional additives,or aqueous of mineral salts, for example. The fluid may be a mist, forexample obtained by suspending water in a gas, or it may be a gas, suchas steam.

From a practical view point, desired cooling of the rolled productsdepends on the cooling devices used, and on suitable choice of thelength and the flow rate characteristics of the cooling means.

The dimensions of the product are known as well as the composition ofthe steel, and thus its continuous cooling transformation diagram,making it possible to determine the conditions to apply for an adequatetreatment of the steel section, among which, the temperature at whichmartensite is formed and the maximum time available for performingsurface quenching to the desired depth.

Based on curves of the temperature gradients in the core and the skin ofthe rolled steel section, the amount of heat to be removed can be aswell as the characteristics of the cooling devices and the flow rates ofthe fluid applied by the cooling devices.

To monitor the formation of the desired microstructures in the differentzones of the steel section, the evolutions of the skin temperature ofthe steel section starting from the end of the martensitic and/orbainitic quenching are being measured. After quenching, the skintemperature rises while the temperature at the core continuouslydecreases after the section has emerged from the last stand of therolling mill. The skin temperature and the core temperature in a givencross-section converge towards a time from where the two curves continuesubstantially parallel to one another. The skin temperature at thispoint is called the “equalization temperature”.

EXAMPLES

Two grades, which compositions are gathered in table 1, were cast insemi-products and processed into steel sections following the processparameters gathered in table 2, going through heating, controlled hotrolling and subsequent water cooling, achieved by quenching andself-tempering.

TABLE 1 Compositions Trial C Mn Si Cu Ni Cr Mo V N Ti Nb Al P S CEV 1 821059 171 170 162 129 49 34 9.6 1 1 3 13 23 0.32 2 98 1559 191 193 117122 35 74 16.2 1 1 13 17 29 0.43

The tested compositions are gathered in the following table wherein theelement content are expressed in thousands of weight percent:

Trial 1 is a comparative example and trial 2 is an example according tothe invention.

TABLE 2 Process parameters Hot rolling Quenching and self-tempering HotSelf- Flange rolling Specific tempering thickness Reheating finish Twater flow Temperature Trial (mm) T (° C.) (° C.) (l/m²/s) (° C.) 1  801150 868 46 640 2 125 1170 893 46 600

Steel semi-products, as cast, were processed under the followingconditions:

The resulting samples were then analyzed and the correspondingmicrostructure elements and mechanical properties were respectivelygathered in table 3 and 4.

TABLE 3 Microstructure and precipitates Hardened zone Core zone TemperedRegular Acicular Pearlite + Trial martensite Bainite Ferrite ferriteBainite 1 33.4% 66.6% 76.7% 5.5% 17.7% 2 56.7% 43.3% 72.4% 0.4% 27.2%

The phase percentages of the microstructures of the obtained steelsection were determined:

The phase percentages in both zones, especially in the core zone, ofsection no 1 are quite similar to section no 2, showing that the impactof the vanadium precipitation strengthening is observed at smallermicrostructural scale.

Precipitation analysis done by TEM examination of carbon extractionreplicas taken from the core zone of the flange thickness of thesection, showed the presence of vanadium precipitates. Fine precipitatesanalysis was performed through TEM thin foil method, which allowedquantifying the mean size and the density of the precipitates.

It was found that precipitates participating in mechanical strengtheningof the section were located in the core zone of the steel sections, inparticular inside the ferrite phase.

FIG. 1 shows the vanadium precipitates mostly having spherical shape,with bigger or smaller size. The bigger size precipitates (typical sizeabout 6 nm in diameter) were mostly randomly distributed. But the fineprecipitates (typical size about 3 nm in diameter) were arranged inregularly spaced bands. It can be seen on FIG. 2 that the microstructureconsists of parallel sheets densely populated with vanadium particles.The sheets appear with a regular spacing.

Regularly Spaced Precipitates

Precipitates characteristics % of precipitates with a Mean density inMean diameter Trial mean diameter below 6 nm ferrite (per mm²) (nm) 1 0%  ≤100 — 2 98%   1213 3.2

Randomly Distributed Precipitates

TABLE 4 Mechanical properties Precipitates characteristics % ofprecipitates with Mean density in Mean Repartition of metallic a meandiameter ferrite diameter elements in precipitates, % Trial below 6 nm(per mm²) (nm) V Cr Mn Fe 1  0% <100 — — — — — 2 70%   880 5.7 66.6 21.94.2 9.3

Mechanical properties of the tested steel were determined and gatheredin the following table:

Yield Strength Tensile strength Trial (MPa) (MPa) CEV (%) 1 398 450 0.322 495 653 0.43

The examples show that the steel sections according to the invention arethe only one to show all the targeted properties thanks to theirspecific composition and microstructures.

Steel sections according to the present invention show excellent valuesof high strength, toughness and good weldability, which is nowadays noteasily achievable. With the steel grade as per the invention, design andconstruction teams involved in large-scale construction projects canbenefit from more efficient structural solutions. The steel section'shigher yield strength enables weight savings and lower transportationand fabrication costs than other commonly-used structural steel grades.And thus, the present invention makes an extremely significantcontribution to construction industry.

1-14. (canceled)
 15. A steel section comprising a web central portionconnected on each side to flange portions having a thickness of at least100 mm, the steel section having a composition comprising, in weightpercentage: C: 0.06-0.16% Mn: 1.10-2.00% Si: 0.10-0.40% Cu: 0.001-0.50%Ni: 0.001-0.30% Cr: 0.001-0.50% Mo: 0.001-0.20% V: 0.06-0.12% N:0.0050%-0.0200% Al≤0.040% P≤0.040% S≤0.030% and optionally including atleast one of the following elements, in weight percentage: Ti<0.005%Nb≤0.05% a reminder being iron and impurities resulting from processing;a microstructure of the steel structure including vanadium precipitates,the vanadium precipitates optionally including at least one metal chosenfrom the group consisting of chromium, manganese and iron, the vanadiumprecipitates being chosen among nitrides, carbides, carbo-nitrides orany combination of them, more than 70% of the vanadium precipitateshaving a mean diameter below 6 nm.
 16. The steel section as recited inclaim 15 wherein such section composition is such that the followingrelationship is fulfilled:0.4≤CEV≤0.6 with CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
 17. The steel sectionas recited in claim 15 wherein the ratio of vanadium to nitrogen amountsis between 2.5 and
 7. 18. The steel section as recited in claim 15wherein the microstructure of the flanges portions includes, fromsurface to core, a hardened zone including tempered martensite andoptionally bainite and a core zone including ferrite and pearlite. 19.The steel section as recited in claim 15 wherein such steel sectionincludes portions having a mean density of the vanadium precipitates ofat least 500 precipitates per mm².
 20. The steel section as recited inclaim 15 wherein at least part of the vanadium precipitates is arrangedin regularly spaced bands.
 21. The steel section as recited in claim 20wherein more than 80% of such regularly spaced precipitates have a meandiameter below 3 nm.
 22. The steel section as recited in claim 21wherein such regularly spaced precipitates include at least vanadium andchromium.
 23. The steel section as recited in claim 15 wherein at leastpart of such precipitates is randomly distributed in a ferrite phaselocated in a core zone of the steel section.
 24. The steel section asrecited in claim 23 wherein more than 80% of the randomly distributedvanadium precipitates have a mean diameter between 3.5 and 6 nm.
 25. Thesteel section as recited in claim 24 wherein the randomly distributedvanadium precipitates include at least vanadium, chromium and iron. 26.The steel section as recited in claim 15 wherein the vanadiumprecipitates are located in a core zone of the steel section.
 27. Thesteel section as recited in claim 15 wherein the flange portions have athickness of at most 140 mm.
 28. A method of manufacturing of a steelsection comprising the following steps: feeding a steel semi-productcomprising, in weight percentage: C: 0.06-0.16% Mn: 1.10-2.00% Si:0.10-0.40% Cu: 0.001-0.50% Ni: 0.001-0.30% Cr: 0.001-0.50% Mo:0.001-0.20% V: 0.06-0.12% N: 0.0050%-0.0200% Al≤0.040% P≤0.040% S≤0.030% and optionally including at least one of the following elements, inweight percentage: Ti≤0.005% Nb≤0.05%  a reminder being iron andimpurities resulting from processing; reheating the steel semi-productat a temperature above 1000° C. and hot rolling the steel semi-productwith a final rolling temperature of at least 850° C., to obtain a hotrolled steel section; and cooling the hot rolled steel section so as toproduce martensitic or bainitic quenching of the surface layer of all orpart of the product, a non-quenched portion of the rolled productremaining at a temperature high enough to enable a self-tempering of thequenched surface layer of martensite or bainite and to transform theaustenite into ferrite and carbides in a core part of the section duringthe subsequent cooling, a maximum temperature of the tempered surface ofthe product after quenching being 450 to 650° C.